1. Field of the Invention
The present invention relates to a flow-thru vacuum bushing for ultracentrifuges and, more particularly, to a vacuum bushing for an ultracentrifuge that minimizes excessive oil loss and allows automatic reclaiming and reusing of a major portion of the uncontaminated oil flow.
2. Description of the Prior Art
It is known in the art of centrifugation to position a rotor in a chamber from which all atmosphere is evacuated. By operating the rotor in a vacuum, substantially higher speeds are permissible than when the rotor is operated in an atmospheric environment.
A typical vacuum bushing-type drive system for a vacuum centrifuge includes a bushing through which the rotor drive shaft passes from atmospheric conditions into the evacuated rotor chamber. The bushing has an annular chamber surrounding the drive shaft, adjacent the atmospheric side of the bushing. The annular chamber is connected to an oil reservoir at atmospheric pressure positioned at a height somewhat above the annular chamber. Thus, oil flows by gravity from the oil reservoir to the annular chamber.
Upon reaching the annular chamber, the oil can flow along the drive shaft in two directions. One path is downward through the clearance between the shaft and the bushing into a drive sump. Since the oil reservoir and drive sump are both at atmospheric pressure, the pressure differential in this path, between the annular chamber and the drive sump, is equal to the difference in height between the oil reservoir and the first annular chamber.
The other path is upward through the clearance between the drive shaft and the bushing into the rotor chamber. Because there is a vacuum in the rotor chamber, the differential head pressure in this path, between the annular chamber and the rotor chamber, is approximately 30 feet. Tests show that about 85% to 95% of the oil flows along this path, due to the greater pressure difference.
This type of system is designed on the premise that the clearance between the rotating drive shaft and the bushing is such that the vacuum pump associated with the rotor chamber can pump more air out of the rotor chamber than can pass through the bushing in the oil. As the maximum speed of a centrifuge is upgraded, the clearance must be opened so that more oil passes through for lubrication. While this does not present a problem with maintaining the vacuum, it does increase the oil usage since the oil that passes through the bushing into the rotor chamber is contaminated and unusable. This is not only an inconvenience to the user but also becomes troublesome to the centrifuge itself due to oil contamination of other systems in the rotor chamber.
Many attempts have been made to solve this problem. It has been often suggested that a completely closed system be provided to recover the oil. However, none of the systems proposed heretofore have been successful for one or more reasons. In some cases, there is still a substantial amount of lost oil. In other cases, even though the excessive oil loss has been minimized, the oil reclaimed has been contaminated and incapable of reuse.